2 - INTERNAL CALIBRATION
Internal calibration of a SAR sensor is necessary to monitor and record variations of the transmit
power and the receiver gain versus temperature and time. In the following, we concentrate on the
X-SAR sensor which is the german-italian contribution to the SIR-C/X-SAR space shuttle mission
[4].
Fig. 1 shows a block diagram of the X-SAR system including calibration relevant sub
systems. The chirp signal is generated in the frequency generation unit (FGU) and upconverted to
9.6 GHz in the frequency conversion unit (FCU). After amplification in the high power amplifier
(HPA), the signal passes the circulator (C) and is transmitted by the antenna. On receipt, the
received power from the antenna goes through the circulator, shutter (S), band-pass filter and limiter
(FL) into the low noise amplifier (LNA). The signal is amplified in the LNA and the subsequent
receiver part (RX), sampled in the analog-digital converter (ADC) and stored on high density digital
tapes. The receiver gain is set by a stepped attenuator (in RX) in steps of 2dB from a maximum
toted gain of 80 dB down to 40 dB.
To monitor essential system parameters (temperature, power, voltage, current), correspond
ing sensors are installed throughout the whole system. The temperature sensors are located at
system elements where the temperature is representative (e.g. SAW filter for RX temperature).
These sensor data are telemetered down as housekeeping (HK) data. It is intended to have these
parameters displayed during the mission in a lm ost real time, to get a first impression of the stability
and an indication of any malfunction. For more accurate monitoring of the sensor a direct control
using special electronics is necessary.
The block diagram in Fig. 1 includes the built-in calibration loops. The TX-calibration
coupler (C3) couples out part of the high power transmit signal. This calibration signed (chirp)
passes a fixed attenuator (A2) and the receiver (excluding the LNA) and is stored in the raw data
stream. The RX-gain for the TX-loop is set to mid gain (60 dB). The RX-calibration uses a part of
the stable FCU-output (leveled output) and checks the whole receiver dynamic range by stepping
the RX-loop attenuator (Al) in steps of 5 dB from 0 to 60dB (calibration level) and the receiver
gain over the whole 40 dB range.
RX- and TX-calibration are performed at the beg innin g and end of each data take according
to the schematic sequence shown in Fig. 2. To take into account variations during data take,
linear interpolation between these two values will be used. The calibration data are formatted like
Figure 1: X-SAR block diagram including calibration loops.
